The present invention relates to novel fluorinated Positron Emission Tomography (PET) biomarkers, and, more particularly, to a fluorinated biomarkers for quantification of epidermal growth factor receptor tyrosine kinase.
Positron Emission Tomography (PET), a nuclear medicine imagine technology which allows the three-dimensional, quantitative determination of the distribution of radioactivity within the human body, is becoming an increasingly important tool for the measurement of physiological, biochemical, and pharmacological function at a molecular level, both in healthy and pathological states. PET requires the administration to the subject of a molecule labeled with a positron-emitting nuclide (radiotracer) such as .sup.15 O, .sup.13 N, .sup.11 C, and .sup.18 F, which have half-lives of 2, 10, 20, and 110 minutes, respectively.
Polypeptides such as growth factors, differentiation factors, and hormones often mediate their pleiotropic actions by binding to and activating cell surface receptors with an intrinsic intracellular protein tyrosine kinase activity. Epidermal growth factor receptor-tyrosine kinase (EGFR-TK) is over expressed in breast cancer and other neoplasia. A suitable radiotracer that binds to EGFR-TK might allow, through a nuclear medicine imaging technique such as PET, the mapping and quantification of this receptor-kinase. This would allow the study of changes in levels of expression of this receptor, including the monitoring of response to hormonal or other chemotherapy, and could lead to better patient management and differentiation in regard to therapeutic course of action.
Recently, .sup.99m Tc-labeled anti EGFR antibody was synthesized and biodistribution and dosimetry studies were performed in humans [1, 2]. However this labeled antibody, similar to other protein radiopharmaceuticals, has high and prolonged retention of radioactivity in the liver which constitutes a major problem for clinical applications. Furthermore, these researchers found that it was difficult to obtain accurate quantification of activity in tumors within normal organs because of varying background activities, particularly in lung lesions where fluid and atelectasis could not be differentiated from tumor.
EGF itself has been labeled for nuclear medicine imaging with gamma emitting nuclides including .sup.99m Tc [3, 4] and .sup.111 In [5, 6], and the positron-emitting nuclide .sup.76 Br [7, 8]. The biodistribution in normal rats of the latter, [.sup.76 Br]EGF (murine), was reported [8], but no other in vivo studies in laboratory animals or humans have been reported.
4-Anilinoquinazolines have been shown to potently and selectively inhibit EGFR-TK activity by binding reversibly to an inner membrane ATP binding site on EGFR-TK, the prototype for such compounds being the small-molecules PD 153035 [9] and AG1478 [10] (see Table 1 below). A report of a radioiodinated analog of PD 153035 including in vitro binding studies in MDA-486 cells has been presented [11]. PD 153035 labeled with .sup.11 C in the 6,7-methoxy groups has been evaluated in rats implanted with human neuroblastoma xenografts (SH-SY5Y) but specific uptake was not determined in a blocking study [12]. PD 153035 was also labeled with .sup.11 C specifically in the 7-methoxy position and biodistribution experiments were performed in normal mice, but uptake specificity could not be demonstrated as administration of an enzyme-blocking dose of PD 153035 caused an increase in tracer uptake in the tissues studied [13]. The same abstract reported the labeling of the 7-(2-fluoroethoxy) PD 153035 analog with .sup.18 F, but no biological experiments with this tracer were described. Additionally, the 2-[.sup.18 F]fluoroethyl group might be subject to a high rate of [.sup.18 F]hydrogen fluoride elimination to give the corresponding alkene ether, potentially resulting in high uptake of .sup.18 F in bone, giving poor in vivo images. Further, these ultra potent (IC.sub.50 &lt;30 pM) inhibitors may only measure flow or permeability surface area rather than biochemical changes [14]. And, PD 153035 has been shown to undergo metabolism to four compounds, two of which have been identified as the 6- and 7-monodemethylated derivatives, compounds which maintain potency for EGFR-TK inhibition [15]. Thus labeling in the 6- or 7-alkoxy groups as described above, would lead to radioactive metabolites that do not bind EGFR-TK.
Another approach to small molecules as EGFR-TK PET tracers are 4-anilinoquinazoline derivatives labeled with .sup.18 F on the aniline ring (see reference [16]). Assuming that metabolism of PD 153035 arylfluoro derivatives is similar to the metabolism of PD 153035 itself, in this approach the monodemethylated derivatives would retain the radiolabel and should maintain affinity for EGFR-TK. While the presence in the blood of three compounds with potential to bind the target would complicate kinetic compartmental modeling, the probability of accumulation of radioactivity in the target would be increased. .sup.18 F's half life, five times longer than the half life of .sup.11 C, affords a wider time-window for PET measurements than .sup.11 C does, possibly allowing the benefit of imaging after disappearance of blood radioactivity and washout of radiotracer nonspecific binding. Since PD 153035 plasma levels decrease from a maximum to 2% of the maximum after approximately 3 hours [15], later imaging, perhaps an hour or more postinjection (virtually impossible with .sup.11 C), may give a more pure signal of EGFR-TK presence. In addition, labeling the aniline ring may result in a more inherently metabolically stable tracer that will lead to low nonspecific uptake of radioactivity in, e.g., bone. Additional related art is disclosed in U.S. Pat. No. 5,710,158; EP 566226B1 and CA 2,086,968.
There is thus a widely recognized need for, and it would be highly advantageous to have, novel fluorinated PET biomarkers devoid of the above limitations.